Reduction of furans

A number of the bicyclic ozonides 12 were prepared in good yield (45-65 %) by diimide reduction of furan singlet oxygenates (Eq. 9) 23>. Again, low temperature were essential because the furan endoperoxides readily transform into 1,2-diacyl-ethylenes. Of course, the bicyclic ozonides 12 can alternatively be prepared via ozonolysis of the appropriate 1,2-disubstituted cyclobutene 24). 

Studies of cathodic reduction have been few. Amusingly, attempted anodic oxidation of the furyl ketone 123 actually resulted in cathodic reduction to the dimer 124 the corresponding ester was oxidized normally, however.301 Sometimes the dimethoxydihydrofurans formed in oxidation processes are reduced in a side reaction leading to the tetrahydrofuran derivatives.302 By using dimethylformamide as solvent instead of the protic solvents used formerly, a Czech group has demonstrated that the cathodic reductions of furans can produce fairly stable anion radicals having ESR spectra which agree well with theory.3023... 

Donohoe reports a novel and unprecedented reductive aldol process involving the Birch reduction of furans and pyrroles (e.g., 62) which presumably generates a dianion 63 and subsequently (after protonation at C-5 by ammonia) an enolate 64. After quenching excess... 

These results mark the very first successful catalytic reduction of furans in high enantioselectivity and greatly expand the methods available for the synthesis of enantiopure heterocycles. 

Reduction of furan and its homologs is readily achieved by catalytic hydrogenation using palladium [419, nickel [420] or Raney nickel [421 as catalysts. Too energetic conditions are to be avoided since hydrogenated furan rings are easily hydrogenolyzed [422,423. ... 

Catalytic reduction of pyrroles gives successively A3-pyrroles and pyrrolidines. Tetrahydrofurans are formed by the catalytic reduction of furans with Raney nickel and hydrogen ring cleavage products... 

Ring opening is common in the alkali metal and liquid ammonia reduction of furans unless an anion stabilizing group is present, so most work has been done with derivatives of furancarboxylic acids. Treatment of furan-2-carboxylic acid with lithium and ammonia at -78 °C followed by rapid addition of ammonium chloride affords 2,5-dihydrofuran-2-carboxylic acid (80%). Reductive alkylation similarly gives 2-alkyl-2,5-dihydrofuran-2-carboxylic acids. This method has been used in a synthesis of rosefuran, the intermediate dihydrofuran (66) being converted into the product (67) by oxidative decarboxylation with... 

Tetrahydrofuran is a commonly used solvent. It is a relatively stable ether and is miscible with both water and organic solvents. It is manufactured on a large scale by dehydration of butane-1,4-diol and catalytic reduction of furan. It is also obtained by base treatment of 4-haloalkan-l-ols and by acidic treatment of alk-3-en-l-ols and alk-4-en-l-ols. 

Alkylfurans may be prepared by decarboxylation of alkylfurancarboxylic acids or by Wolf-Kishner reduction of furan aldehydes and ketones. The direct alkylation of furan is also useful in obtaining alkylfurans. 

In the high-pressure apparatus of Adkins, 120 g. of furan can be hydrogenated with 10 g. of Raney nickel catalyst in a single run. Using pressiures of 100-150 atm. and a temperatiure range of 100-150° the reduction is extremely rapid and is strongly exothermic. Platinum oxide-platinum black is not a satisfactory catalyst for the reduction of furan. ... 

Tetrahydrofuran has been prepared by the reduction of furan in the vapor phase with a nickel catalyst at 170°, in butyl alcohol at 50° with Raney nickel catalyst/ and with palladous oxide-palladium black in the absence of a solvent. ... 

Although normally very difficult, the reduction of furans to 4,5-dihydrofurans can be accomplished by trapping, and later regenerating, the dihydro product (equation 11) as the trifluoroacetate (9). ... 

A very convenient and versatile new reduction method, which has been intensively studied for the reduction of thiophenes (vide infra), makes use of the combination of a trialkylsilane and an acid, usually trifluoroacetic acid (TFA). This so-called ionic hydrogenation provides a novel reduction of furans to tetrahydrofurans (Scheme 3). Although Bolestova et al. find that the conditions are critical in order to avoid polymerization, the yields of tetrahydrofurans can be quite good. The reduction is accelerated by the presence of boron trifluoride etherate but fails with carbonyl-substituted furans, such as methyl pyro-mucate. ... 

Several furan aldehydes and ketones without a heterocyclic nitro group have been investigated polarographically and the intervention of ketyl radicals or their conjugate acids confirmed. Their involvement in amalgam reductions of furan carbonyl derivatives was also shown. 

Natural product synthesis as a test of newly developed methodologies, for example, partial reduction of furans and pyrroles, nucleophilic addition to pyridinium salts. Os-catalyzed oxidative cyclization for the synthesis of tet-rahydrofurans and pyrrolidines, tethered aminohydroxylation of aUcenes, and ring-closing- and cross-metathesis for the constmction of heteroaromatics 12CC11924. 

X, and Kondo, A. (2013) Reduction of furan derivatives by overexpressing NADH-dependent Adhl improves ethanol fermentation using xylose as sole carbon source with Saccharomyces cerevisiae harboring XR-XDH pathway. Appl. Microbiol Biotechnol, 97, 2597-2607. 

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